Expression Analysis of High Zinc Rice Breeding Lines Using Known Homeostasis Genes Involved in Iron and Zinc Acquisition and Translocation

A number of homeostasis genes for iron uptake, transport and translocation have been well studied in rice.  To examine the levels of expression of iron and zinc homeostasis genes in rice genotypes, a set of 25 validated genes were used in this study. The seven rice genotypes examined in this study are composed of advanced breeding lines submitted for release in the national systems of Bangladesh through the Bangladesh Rice Research Institute (BRRI) and Philippines through the International Rice Research Institute (IRRI), and IR64, an indica megavariety which was one of the breeding parents of these lines was used as the check variety. These lines have varying iron and zinc levels in the grain which ranges from 2-8 ppm and 15-25 ppm, respectively. The experiment was conducted at IRRI from June 2013 to April 2014.  The individual plants were grown to reproductive stage on a 120-cm PVC pipes lined with plastic sheets to facilitate more efficient collection of the roots from each plant. The different tissues, namely root, flag leaf and milky endosperm were collected at 15 days after pollen dehiscence. Expression analysis was done using quantitative PCR and data was analyzed using the 2-delta delta Ct method, with IR64 as the control and triose phosphate isomerase (TPI) gene as the endogenous check Two high zinc BRRI lines and one of the IRRI high zinc genotype exhibited more than 10-fold increase in expression of OsZIP5 in the roots. These three genotypes have the highest grain zinc level at 25ppm. These two genes are both involved in the uptake of zinc as bound Zn(II)-DMA to be transported in the plant vesicles or plasma membranes.  We also see a 5-fold increase in FRO1 levels in the flag leaf of the two IRRI high zinc lines.  These results concur to earlier reports by Ishimaru et al and Bashir et al where they observed different  expression levels of some of the transporter and homoeostasis genes in the different rice tissues.